1. Field
The present invention relates to an azo pigment or tautomer thereof, a process for producing the same, a pigment dispersion, a coloring composition, an inkjet recording ink, a coloring composition for color filter, and a color filter.
2. Description of the Related Art
In recent years, materials for forming color images have been particularly predominant as image recording materials. More specifically, inkjet system recording materials, thermal transfer system recording materials, electrographic system recording materials, transfer type silver halide photosensitive materials, printing inks, recording pens, and the like have been used actively. In image sensors such as CCDs for photographing equipment and in LCDs and PDPs for display, color filters are used for recording or reproducing a color image. In these color image recording materials or color filters, colorants (dyes or pigments) of three primary colors in a so-called additive color mixing process or subtractive color mixing process have been used in order to display or record full-color images. At present, however, there is no colorant having absorption characteristics capable of realizing a preferred color reproduction region and at the same time, having sufficient fastness to withstand various using conditions or environmental conditions. Thus, there is an eager demand for the improvement of it.
Dyes or pigments to be used for the above-described uses are required to have in common the following properties. Described specifically, they are required to have absorption characteristics preferable in view of color reproduction and have good fastness under the environmental conditions under which they are used, for example, light fastness, heat resistance, and fastness to an oxidizing gas such as ozone. In addition, when a colorant is a pigment, the pigment is further required to be substantially insoluble in water or an organic solvent, to have good fastness to chemicals, and not to lose preferable absorption characteristics in the molecularly dispersed state even if it is used as particles. Although the required properties described above can be controlled by adjusting the intensity of an intermolecular interaction, these two requirements are in a trade-off relation with each other, thus being difficult to satisfy both of them simultaneously.
In using a pigment as the colorant, the pigment is additionally required to have the following properties, that is, having a particle size and a particle shape necessary for realizing desired transparency, having good fastness under environmental conditions under which it is used, for example, good light fastness, heat resistance, and fastness to an oxidizing gas such as ozone, and also good fastness to organic solvents or chemicals such as a sulfurous acid gas, and being dispersed into fine particles in a used medium while keeping a stable dispersion state.
Compared with a dye required to have performances as colorant molecules, the pigment is required to have more performances. The pigment is required to satisfy not only performances as colorant molecules but all of the above-described requirements as a solid which is an aggregate of colorant molecules (a dispersion of fine particles). As a result, a group of compounds which can be used as pigments is severely limited, in comparison with dyes. Even when pigments are derived from high-performance dyes, only a few of the resulting pigments can satisfy the requirement for the performances as a dispersion of fine particles. Such pigments cannot be developed easily. This can be confirmed from the fact that the number of pigments registered in Color Index is below 1/10 of the number of dyes.
Azo pigments are excellent in hue and tinctorial strength which are color characteristics so that they have widely been used in printing inks, inks for inkjet recording, and electrophotographic materials. Of these widely-used azo pigments, the most typically used ones are yellow diarylide pigments and red naphthol azo pigments. Examples of such diarylide pigments include C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, and C.I. Pigment Yellow 17. Examples of the naphthol azo pigments include C.I. Pigment 208 and C.I. Pigment 242. These pigments are however inferior in fastness, particularly light fastness so that when a printed matter with them is exposed to light, they are decomposed, thus being not suited for long-term storage of the printed matter.
When a full color is expressed using three colors including yellow, magenta, and cyan or four colors including black in addition to these three colors in accordance with a subtractive color mixing process, the gray balance of a printed matter changes with the passage of time by using only one pigment inferior in fastness, or color reproducibility upon printing is deteriorated by using a pigment inferior in color characteristics. There is therefore a demand for a pigment and a pigment dispersion having both color characteristics and fastness in order to obtain a printed matter while keeping high color reproducibility for a long period of time.
JP-A-2010-47750 discloses an azo pigment excellent in color characteristics such as tinctorial strength and hue and at the same time excellent in durability such as light fastness.
Most of the typical organic pigments are polymorphic. Such pigments are known to have two or more crystal forms, though they have the same chemical composition.
Some of organic pigments, for example, azo pigments, can be provided in the form of finely granulated particles by selecting proper reaction conditions upon synthesis. For example, copper phthalocyanine green pigments are provided as a pigment by subjecting very fine and aggregated particles, which have been prepared upon synthesis, to grain growth and granulation in subsequent steps. Copper phthalocyanine blue pigments are provided as a pigment by subjecting coarse and uneven particles, which have been prepared upon synthesis, to miniaturization and granulation. For example, a diketopyrrolopyrrole pigment is usually synthesized by reacting a succinic diester with an aromatic nitrile in an organic solvent (refer to, for example, JP-A-58-210084). The crude diketopyrrolopyrrole pigment is heat treated in water or an organic solvent, followed by pulverization such as wet grinding into a form suited for use (refer to, for example, JP-A-5-222314). C.I. Pigment Red 254 is known to have α- and β-crystal forms (refer to, for example, JP-A-8-48908). Further, C.I. Pigment Yellow 181 which is an azo pigment is known to have several crystal forms (refer to, for example, US 2008/0058531).